The invention relates generally to cooling methods and systems employed in gas cooled generators, for example, and, more particularly to cooling the endwinding turns of the rotor field windings in such machines.
A generator converts mechanical energy into electrical energy. The mechanical energy from a gas or steam turbine, for example, is supplied to a rotating field, or rotor, that spins inside a stationary armature, or stator. A generator rotor comprises a large iron forging in which axial slots are filled in circumferential direction. Concentric rectangular copper coils are mounted in these slots. Adjacent coils are electrically connected. Each coil consists of a stack of copper turns. In all, the copper coils and turns form a single circuit. Strips of insulation are placed between turns to prevent electrical shorting of the circuit. The end portions of the coils (commonly referred to as endwinding region), which are beyond the support of the main rotor body, are typically supported against rotational forces by a retaining ring enveloping the coils. In the endwinding region, the coils run partially axially along the length of the rotor and partially circumferentially to connect a set of copper turns in one slot to a set of copper turns in a different slot. Spacer blocks are placed at different locations between the coils in the endwinding region to keep the coils separated from each other and provide mechanical support.
Heat is generated in the copper turns due to Ohmic losses related to electric currents. The maximum current that can be supplied to the turns is often limited by the ability to cool the rotor field windings effectively as there are limits placed on the average and maximum temperature the strips of insulation between the copper turns can obtain. By making improvements to the cooling methods, one can increase the current to the rotor field windings and thus increase the power output and power density of a generator. For high power density generators, the effective cooling of the rotor endwinding region is often the limiting factor in the rotor cooling.
Without any cooling enhancements in the rotor endwinding region, in its most basic configuration, coolant gas is supplied to an annular region between the bottom of the coils and the rotor spindle. The coolant gas flows from the outer most coils farthest away from the rotor body center towards the innermost coil where it enters the rotor body. Few coil turns at the bottom may be effectively cooled by this coolant flow. In addition there are open spaces between the coils and the spacer blocks, which are called cavities. In its basic configuration these cavities are open to the annular gap on one side and closed on the other side by the enveloping retaining ring. Coolant flow can enter these cavities from the bottom, cool the sides of the coils and return again to the annular gap at the same side where the flow entered. As the cavity is closed on one side, this is not the most effective way of cooling the endwinding region.
As the power density of the rotor increases, one needs to provide additional means of cooling the endwinding region effectively. In one attempted solution, passages are created in the retaining ring, either in the radial or axial direction to vent the flow that enters a cavity into the airgap, the space between the rotor and stator. However, the retaining ring is one of the highest stressed areas in the generator and drilling additional openings is not always a feasible solution as mechanical stresses tend to increase.
Accordingly, there is a need for a cooling system for cooling rotor endwindings with improved heat removal capability.